Transforming growth factor-β (TGF-β) regulates proliferation, morphogenesis, and functional differentiation in the mammary gland and plays complex roles in mammary tumorigenesis. Here we show that the signaling mediators Smad1–Smad5 are expressed at all stages of mammary gland development. To begin to investigate which Smads mediate which TGF-β responses, we have analyzed mammary gland development in Smad3 null mice. Smad3 null virgin females showed delayed mammary gland development. However, this phenotype was secondary to ovarian insufficiency because Smad3 null mammary epithelium developed normally in hormonally supplemented Smad3 null mice or when transplanted into wild-type hosts. Absence of Smad3 had no effect on the ability of TGF-β to inhibit the growth of mammary epithelial cells in culture, and no compensatory changes in expression or activation of Smad2 were seen in the Smad3 null epithelium. A small but significant decrease in apoptotic cells was seen in involuting glands from Smad3 null transplants. The results suggest that epithelial Smad3 is dispensable for TGF-β effects on proliferation and differentiation in the mammary gland, but that it contributes in a nonredundant manner to the induction of apoptosis.